The present invention relates generally to radar antennas, and more specifically the invention pertains to a process for measuring the antenna pattern of a phased array antenna.
Traditional antenna measurements use 2D.sup.2 /.lambda. as a criteria for measuring an antenna under test (AUT) in the far field, where R is the distance from the AUT to a point in space, D is the aperture size of the AUT, and .lambda. is the operating wavelength. Problems arise when D becomes large, which increases the distance R, therefore more real estate is needed to measure the far field antenna pattern.
Development over the years has led to techniques to measure the far field pattern of the AUT on smaller ranges. Using a small range reduces outside interference, eliminates testing time due to poor weather conditions, and provides the option of doing classified testing. Many of the previous smaller range techniques eliminate or lessen these problems. There are however, other problems with these techniques. For example, transformation from near field probing to the far field requires many sample points, the large size and high surface tolerance requirements of a reflector restricts the use of a compact range.
There remains a need for a technique to measure the far field antenna pattern over a 100.degree. angular sector that is 1/10 the size of the conventional far field range, but with errors comparable to the errors one would expect from a far field range.
The task of reducing the range requirements while measuring the far field antenna patterns of large antenna arrays is alleviated, to some extent, by the systems disclosed in the following U.S. Pat. Nos., the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 4,998,112 issued to Franchi et al; PA1 U.S. Pat. No. 4,661,820 issued to Pouit et al; PA1 U.S. Pat. No. 4,553,145 issued to Evans; and PA1 U.S. Pat. No. 3,879,733 issued to Hansen et al.
The patents identified above, relate to methods for measuring antenna patterns. In particular, the Franchi et al. patent describes a method for measuring the far field antenna pattern of a large phased array antenna by applying a correction factor to the antenna phase shifters. The antenna is electronically steered to each of its scan angles, and a particular correction factor is applied to each radiating element to correct the far field antenna pattern to the shortened far field range.
The Pouit et al. patent is directed to a method and apparatus for determining the radiation pattern of an antenna from measurements in the near-field. The antenna is illuminated by a transmit antenna having wavefronts in the shape of circular cylinders. The radiation pattern of the antenna is expressed by an integral transform of a unidimensional function which is mathematically solved.
The Evans patent relates to a method of forming the far field beam pattern of an antenna at short ranges. The antenna beam pattern is rotated through a rotational angle about the line of sight of a source of radar signaling located at a near-field range away from the antenna. Measurements of the antenna field are taken at predetermined angular increments. The far-field beam pattern of the antenna is formed by deriving discrete values thereof for corresponding angle increments along the angular rotation. The discrete beam pattern value corresponding to each selected angular increment is derived as the weighted summation of the multiplicity of antenna field measurements associated therewith.
The Hansen et al. patent describes an apparatus and method for obtaining far-field antenna patterns. Actual phase and amplitude measurements are obtained in the near-field, and then processed in a digital computer. Theoretical Z-matrix numerical techniques are used to calculate the far-field patterns by solving the matrix for the current distribution on the structure.
While the above-cited references are instructive, the need remains to measure the far field patterns of phased array antennas in a manner that reduces the range requirements. The present invention is intended to satisfy that need.